Believe me with started or adult birds, not chicks, can be very warm in those boxes regardless of how cold it is. Baby chicks, I would say, wait a while. You have to think about birds and other animals out on the tarmac and between flights and how long they will be sitting out on the trailer. To prepare ahead of time is an ounce worth of prevention!
If it is going to be snowing or ice, delay shipping.
If it is nice cold day, they will be fine.
Pack apples, pears and grapes for them. More than they need for one day if they ended up having a hold over.
UPS, Fed EX do not ship birds except for the routing or direct flights. Fed EX has been known to ship birds but they will not deliver them.....USPS are the ones to deliver them and have a contract with Fed EX to do some of their overload or special shipments. My post office uses the Fed Ex's schedules for flights from a major city and how they would truck them there.